1. Field of the Invention
This invention relates to polyphenylene ether resin thermoplastics and more particularly to blends of such resins which are useful as electrical insulating coatings.
2. Brief Description of the Related Art
Polyphenylene ethers are a widely used class of thermoplastic engineering resins characterized by excellent hydrolytic stability, dimensional stability, toughness, heat resistance and dielectric properties. However, they may be deficient in certain other desired properties such as workability and flame retardance. Therefore, there is a continuing need for means of modifying polyphenylene ethers to improve certain properties, particularly flexibility, tensile elongation and strippability to enhance their use as wire coatings and in particular electrical insulation coatings for electrical conductors.
Polyphenylene ether (PPE) based compositions useful as dielectric insulators sometimes include polyolefins, and exhibit improved impact strength and chemical resistance. However, the inclusion of olefinic components can decrease the flame retardance of the overall composition, thereby necessitating the addition of special additives to regain this important property. As an illustration, U.S. Pat. Nos. 4,808,647 and 4,945,018 (V. Abolins et al.) describe the use of a multi-component flame retardant composition which includes an organic phosphate and a brominated material such as a tetrabromobisphenol-A oligomeric polycarbonate.
While the compositions described in the Abolins patents exhibit excellent properties suitable for many applications, it is often desirable to minimize or eliminate the use of bromine or other halogen-containing additives in compositions designed for certain products. For example, the wire insulation compositions mentioned above frequently cannot contain halogen additives because they could form corrosive compounds when exposed to flame, thereby resulting in considerable damage to surrounding electrical equipment. Nevertheless, any modification of such an insulation composition must not result in significant decreases in the other important properties necessary for very durable products, e.g., flexibility and "stretchability".
It is therefore apparent that a need still exists for polyphenylene ether-based compositions that are flame retardant, flexible, and noncorrosive. In particular, a need exists for halogen-free wire insulation compositions which are flame retardant, and which exhibit very high tensile elongation and flexibility. The compositions of the present invention obviate the need for halogen-containing flame retarding additives, employing instead a phenylsiloxane fluid ingredient which imparts a degree of flame retardance to articles made from the compositions of the invention.
A number of PPE based compositions have included as an additive polyorganosiloxanes; see for example the descriptions found in the U.S. Pat. No. 3,737,479 (Haaf). Lovgren et al, U.S. Pat. Nos. 4,446,090 and 4,487,858 show that high viscosity silicone fluids have been blended with thermoplastic polymers to produce flame retardant thermoplastic materials. However, since the polyorganosiloxanes also function as an impact-modifier in blends thereof with thermoplastic resins, their use as plasticizers was not seriously considered.